1. Field of the Invention
The present invention relates to a focus detection device suitable for a focus detection apparatus of a camera.
2. Description of the Related Art
A focus detection apparatus equipped with a phase difference CCD focus detection device is provided for a conventional AF single-lens reflex camera. The CCD focus detection device includes line sensors and monitor sensors corresponding with a photographic optical system and focus detection areas. Each of the monitor sensors receives light from a photographic subject (object) in approximately the same area as that of the adjacent CCD line sensor to output a monitor signal in accordance with the amount of received light. Thereafter, the monitor signal is amplified in an auto gain controller. When the amplified monitor signal reaches a predetermined integration termination value, the integration for the corresponding CCD line sensor is terminated.
However, it has been discovered that the integrated value of the CCD line sensor and the monitor output of the monitor sensor are not linearly proportional to each other. Specifically, the CCD focus detection device is proven to have a characteristic that, as the integration time is prolonged (the brightness of a photographic subject becomes lower), an appropriate integrated value of the CCD line sensor is decreased as compared with the monitor signal of the monitor sensor. Therefore, in the case of high brightness, the integrated value of a high-brightness portion of the CCD line sensor exceeds a saturated integrated value, preventing accurate phase difference measurement. On the other hand, in the case of low brightness, the integrated value becomes smaller than the appropriate integrated value, preventing efficient use of a dynamic range of the CCD line sensor. As a result, a contrast cannot be obtained. As described above, in the focus detection device using the conventional CCD line sensors, the output of the CCD line sensor sometimes becomes non-uniform depending on the brightness of the object, preventing accurate measurement.